Looking for experience

This buff Orpington chick came from Tractor Supply about 3 weeks ago. She has remained smaller than the other two that came with her. But she was eating and drinking and pooping. The last 36 hours she’s gone downhill. I just got nutri drench and have given her a bit of that directly by mouth. This looks like seizures to me but she rallied this afternoon after looking like she would die last night. So maybe it’s not seizures. Primarily looking to see if you have a good guess as to what this is. I’m just trying to help an elderly neighbor by doing intensive care for the chick.